Patent application title: POLYMER MICROPARTICLES AND PRODUCTION METHOD FOR THE SAME

Abstract:

A method for producing polymer particles which includes: forming polymer
particles by heterogeneous polymerization of a monomer in a supercritical
fluid and/or subcritical fluid, wherein a radical polymerization
initiator (I) having in a molecular at least a group having
radical-generation capability and a group having affinity for the
supercritical fluid and/or subcritical fluid, and a radical
polymerization initiator (II) are used in combination.

Claims:

1. A method for producing polymer particles comprising:forming polymer
particles by heterogeneous polymerization of a monomer in a supercritical
fluid and/or subcritical fluid,wherein a radical polymerization initiator
(I) having in a molecular at least a group having radical-generation
capability and a group having affinity for the supercritical fluid and/or
subcritical fluid, and a radical polymerization initiator (II) are used
in combination, and the radical polymerization initiator (I) has a
number-average molecular weight more than 50,000.

3. The method according to claim 1, wherein the group having affinity for
the supercritical fluid and/or subcritical fluid is a group having an
organopolysiloxane skeleton.

4. The method according to claim 1, wherein the group having
radical-generation capability is azo group.

5. The method according to claim 1, wherein the radical polymerization
initiator (I) has a number-average molecular weight of 70,000 to 90,000.

6. The method according to claim 1, wherein the radical polymerization
initiator (I) is a polymer azo-based polymerization initiator having an
organopolysiloxane skeleton.

7. The method according to claim 6, wherein the polymer azo-based
polymerization initiator has a structure in which azo groups and the
groups having the organopolysiloxane skeleton are repeatedly bonded
together.

8. The method according to claim 6, wherein the polymer azo-based
polymerization initiator has a structure represented by the following
General Formula (1): ##STR00004## where R1 to R5, R7, and
R9 to R12 each represent a hydrocarbon group having 1 to 4
carbon atoms; m and n each represent an integer indicating the number of
repeating units; and R6 and R8 each represent cyano group.

9. The method according to claim 8, wherein in General Formula (1) m
represents an integer of 50 to 200, and n represents an integer of 3 to
15.

10. The method according to claim 1, wherein the radical polymerization
initiator (II) is an azo-based initiator.

11. The method according to claim 1, wherein the monomer contains an
aromatic vinyl monomer and/or a derivative thereof.

12. Polymer particles obtained by a method for producing polymer
particles, which method comprises forming polymer particles by
heterogeneous polymerization of a monomer in a supercritical fluid and/or
subcritical fluid, wherein a radical polymerization initiator (I) having
in a molecular at least a group having radical-generation capability and
a group having affinity for the supercritical fluid and/or subcritical
fluid, and a radical polymerization initiator (II) are used in
combination.

13. The polymer particles according to claim 12, wherein the polymer
particles are polyhedral polymer particles.

Description:

BACKGROUND OF THE INVENTION

[0001]1. Field of the Invention

[0002]The present invention relates to polymer microparticles prepared by
heterogeneous polymerization of a monomer in a supercritical fluid and/or
subcritical fluid, and a production method for the same. The polymer
microparticles promise to be used as constituent materials of developers
in electrophotography, printing inks, building paints, and cosmetics.

[0003]2. Description of the Related Art

[0004]There have been proposed microparticle production methods that
involve heterogeneous polymerization of a monomer in supercritical carbon
dioxide, and emulsion polymerization, dispersion polymerization,
suspension polymerization, etc., are well known in the art. Among other
methods, heterogeneous polymerization conducted in supercritical carbon
dioxide is advantageous over conventional heterogeneous polymerization
conducted in water or organic solvent, since it can (1) achieve
simplification of solvent removal (drying) step after polymerization, (2)
requires no waste solvent treatment, and (3) uses no toxic organic
solvents. For these reasons, heterogeneous polymerization is widely used
for the preparation of microparticles from a monomer, and the resultant
microparticles are used for instance in the above-described applications.
Nevertheless, many of the conventional preparation methods involving
heterogeneous polymerization in supercritical carbon dioxide require
surfactant upon granulation; therefore, an optional surfactant need to be
prepared in advance for each type of the monomer to be used. When polymer
particles are to be prepared from multiple types of monomers by
heterogeneous polymerization, different surfactants need to be previously
prepared for different types of monomers, requiring multiple devices and
resulting in prolonged lead time, increased production steps, and low
yields. These disadvantages increase production costs, and therefore,
there remains a need in the art to overcome the disadvantages.

[0005]Specifically, the first objective in the art was to obtain polymer
particles without having to prepare different surfactants for different
types of monomers.

[0006]The second objective was to improve compatibility between the
obtained particles and organic medium, resin, metal, etc. Poor
compatibility is attributed to the fact that the obtained particles have
substantially smooth surface and are substantially spherical. Thus, there
often occurs a situation where polymer particles combined with organic
medium for use as a film or paint come off from the medium, a situation
where resin or metal covering the particle surface easily come off, and
so forth.

[0007]To achieve the first objective, Japanese Patent Application
Laid-Open (JP-A) No. 2002-179707 discloses a method of preparing
submicron resin particles by polymerization of an acrylic monomer while
using a polymerization initiator having a polydimethylsiloxane skeleton
(product name: VPS-501 (Wako Pure Chemical Industries, Ltd.)) This
method, however, is encountered with difficulty in obtaining discrete
particles since they undergo flocculation and aggregation. In particular,
it has been difficult with this method to obtain micron resin particles
since they tend to be flocculated and aggregated extensively. Moreover,
polymerization particles obtained with this method generally have a
weight-average molecular weight (Mw) of 100,000 to 600,000 and a
number-average molecular weight (Mn) of 50,000 to 300,000. Thus, with
this method, it has been difficult to obtain low-molecular-weight
polymerization particles with a molecular weight of around 3,000 to
50,000 suitable for use as toner or image forming particles. The reason
for this is that low-molecular-weight polymerization particles are prone
to fluidization due to plasticization by means of supercritical carbon
dioxide and thus are more likely to undergo flocculation and aggregation
than high-molecular-weight polymerization particles. Namely, it has been
very difficult to prepare micron polymerization particles with a low
molecular weight without causing flocculation and aggregation.

[0008]As another approach to achieve the above first objective, Shishido
et al of Yamagata University proposes, on page 152 of their book titled
"Supercritical Fluid and Nanotechnology," a method of preparing polymer
particles by using, without any surfactant, acrylonitrile as a monomer in
which the obtained polymerization particles are insoluble. In this
method, however, unwanted particle flocculation occurs due to the absence
of surfactant. Moreover, this method is significantly limited in
applicability since it is required to exploit the nature of the resultant
polymerized particles that they are insoluble in the monomer used;
therefore, employable monomers are limited to acrylonitrile, etc, and
this method cannot be used for the production of general-purpose polymers
such as polystyrene and methyl methacrylate (MMA). U.S. Pat. No.
5,552,502 issued to Odell et al studies on deposition polymerization in
supercritical carbon dioxide containing sulfur dioxide. However, sulfur
dioxide presents safety problem since it is toxic and corrosive to the
device. U.S. Pat. No. 5,688,870 issued to Wilkinson et al tries to
prepare particles with improved water dispersibility by preparing resin
particles from a polymerizable monomer in supercritical carbon dioxide
using a silicone surfactant and by forming a hydrophilic shell layer on
the particle surface. However, this method requires in-advance
preparation of surfactants, i.e., separate steps for development and
preparation of surfactants optimal for the intended type of polymer
particles (i.e., monomer types). Thus, it is quite challenge to improve
the above-noted production process.

[0009]To achieve the second objective attempts have been made to modify
particle surface for increased chemical affinity, but have met with
limited success. Another possible physical method for obtaining anchor
effects on the particle surface is to obtain particles by pulverization.
However, pulverization results in generation of particles with a broad
particle size distribution--from coarse particles to finely divided
particles. This makes classification indispensable, which is undesirable
in view of productivity and costs. Also, pulverization may result in too
large variations in particle shape.

[0010]In-water polycondensation has been contemplated as a method of
improving the anchor effect by roughening the particle surfaces for
increased specific surface areas. This method, however has disadvantages
such as low monomer selectivity and generation of large volumes of waste
water.

BRIEF SUMMARY OF THE INVENTION

[0011]A first object of the present invention is to provide an efficient
method for producing polymer particles in a supercritical fluid and/or
subcritical fluid in parallel with a polymer surfactant in a single pot,
without having to previously prepare a surfactant according to the type
of monomer, and to provide micron polymerized particles with a low
molecular weight without causing flocculation and aggregation.

[0012]A second object of the present invention is to provide a technology
of producing polyhedral microparticles by heterogeneous polymerization
using as a solvent a supercritical fluid and/or subcritical fluid.

[0013]The foregoing objects aim to increase the anchor effect of the
surfaces of particles that are used in any of the foregoing applications,
to prevent particle detachment or separation and to increase the adhesion
between particles and covering material.

[0014]The inventors conducted extensive studies and succeeded in providing
a method for producing polymer particles by heterogeneous polymerization
of a monomer in a supercritical fluid and/or subcritical fluid, wherein a
radical polymerization initiator (I) having in a molecular at least a
group having radical-generation capability and a group having affinity
for the supercritical fluid and/or subcritical fluid, and a radical
polymerization initiator (II) having a structure different from that of
the radical polymerization initiator (I) are used in combination with the
monomer whereby the necessity of preparation and addition of a surfactant
according to the type of the monomer are obviated, and providing micron
polymerized particles with a low molecular weight without causing
flocculation and aggregation. The underlying mechanism for this is as
follows. This method first generates polymer radicals by thermal
decomposition of the radical polymerization initiator (I). The radicals
are then reacted with the monomer to produce a polymer surfactant that
acts on that monomer. This polymer surfactant accelerates formation of
polymer particles by heterogeneous polymerization. Specifically, by
combining a monomer, radical polymerization initiator (I) and radical
polymerization initiator (II), heterogeneous polymerization proceeds in
parallel with synthesis of a surfactant that acts on the monomer, whereby
polymer particles are obtained.

[0015]In this way this approach overcome the forgoing problem pertinent in
the art, i.e., eliminated the need of separately preparing surfactants
prior to granulation, and made it possible to obtain polyhedral polymer
microparticles, which is the second objective described above.

[0016]In particular, the inventors found that when a polymer azo-based
polymerization initiator having the following General Formula (1) is used
as the radical polymerization initiator (I), this optimally results in
the formation of polyhedral microparticles.

##STR00001##

[0017]where R1 to R5, R7, and R9 to R12 each
represent a hydrocarbon group having 1 to 4 carbon atoms; m and n each
represent an integer indicating the number of repeating units; and
R6 and R8 each 10 represent cyano group.

[0018]The present invention has been accomplished based on the discovery
of the inventors, and means of solving the foregoing problem are as
follows:

[0019]<1> A method for producing polymer particles including:
forming polymer particles by heterogeneous polymerization of a monomer in
a supercritical fluid and/or subcritical fluid, wherein a radical
polymerization initiator (I) having in a molecular at least a group
having radical-generation capability and a group having affinity for the
supercritical fluid and/or subcritical fluid, and a radical
polymerization initiator (II) are used in combination, and the radical
polymerization initiator (I) has a number-average molecular weight more
than 50,000.

[0021]<3> The method according to <1> or <2>, wherein
the group having affinity for the supercritical fluid and/or subcritical
fluid is a group having an organopolysiloxane skeleton.

[0022]<4> The method according to any one of <1> to <3>,
wherein the group having radical-generation capability is azo group.

[0023]<5> The method according to any one of <1> to <4>,
wherein the radical polymerization initiator (I) has a number-average
molecular weight of 70,000 to 90,000.

[0024]<6> The method according to any one of <1> to <5>,
wherein the radical polymerization initiator (I) is a polymer azo-based
polymerization initiator having an organopolysiloxane skeleton.

[0025]<7> The method according to <6>, wherein the polymer
azo-based polymerization initiator has a structure in which azo groups
and the groups having the organopolysiloxane skeleton are repeatedly
bonded together.

[0026]<8> The method according to <6> or <7>, wherein
the polymer azo-based polymerization initiator has a structure
represented by the following General Formula (1):

##STR00002##

[0027]where R1 to R5, R7, and R9 to R12 each
represent a hydrocarbon group having 1 to 4 carbon atoms; m and n each
represent an integer indicating the number of repeating units; and
R6 and R8 each represent cyano group.

[0028]<9> The method according to <8>, wherein in General
Formula (1) m represents an integer of 50 to 200, and n represents an
integer of 3 to 15.

[0029]<10> The method according to any one of <1> to
<9>, wherein the radical polymerization initiator (II) is an
azo-based initiator.

[0030]<11> The method according to any one of <1> to
<10>, wherein the monomer contains an aromatic vinyl monomer and/or
a derivative thereof.

[0031]<12> Polymer particles obtained by the method according to any
one of <1> to <11>.

[0032]<13> The polymer particles according to <12>, wherein
the polymer particles are polyhedral polymer particles.

[0033]The present invention can provide polymer particles with diameters
of several micrometers offering a uniform particle size distribution and
a production method for the same, by heterogeneous polymerization of a
monomer in a supercritical fluid and/or subcritical fluid by using in
combination a radical polymerization initiator (I) having in a molecular
at least a group having radical-generation capability and a group having
affinity for the supercritical fluid and/or subcritical fluid, and a
radical polymerization initiator (II).

[0041]Hereinafter, the inventive polymer particles and production method
for the same will be described in more detail with reference to specific
embodiments, which however shall not be construed as limiting the scope
of the present invention. It should be understood that any modification,
alteration, and substitution can be anticipated and expected from those
skilled in the art without departing from the teachings of the present
invention.

(Production Method for Polymer Particles)

[0042]The inventive production method for polymer particles uses a radical
polymerization initiator (I) having in a molecular at least a group
having radical-generation capability and a group having affinity for a
supercritical fluid and/or subcritical fluid, and a radical
polymerization initiator (II).

<Radical Polymerization Initiator (I)>

[0043]Radical polymerization initiator (I) includes in a molecular at
least a group having radical-generation capability and a group having
affinity for a supercritical fluid and/or subcritical fluid (e.g.,
supercritical carbon dioxide).

[0046]Examples of the group having radical-generation capability include,
for example, azo group, peroxide group, and hydroperoxide group.

[0047]Preferably, radical polymerization initiator (I), or polymer
surfactant precursor, is a polymer azo-based polymerization initiator
having the following General Formula (1):

##STR00003##

[0048]where R1 to R5, R7, and R9 to R12 each
represent a hydrocarbon group having 1 to 4 carbon atoms; m and n each
represent an integer indicating the number of repeating units; and
R6 and R8 each represent cyano group.

[0049]In General Formula (1), m represents an integer of 50 to 200,
preferably 100 or more, and more preferably 135; and n represents an
integer of 3 to 15, preferably 5 or more, and more preferably 7 to 9.

[0050]Moreover, examples of R1 to R5 and R7 include, for
example, methyl group, ethyl group, n-propyl group, i-propyl group,
n-butyl group, sec-butyl group, tert-butyl group, and substituted or
non-substituted phenyl group. Examples of R9 to R12 include,
for example, methylene group, ethylene group, propylene group, and
butylene group. In particular, it is preferable to employ a compound in
which R1 to R5 and R7 are methyl group, R6 and
R8 are cyano group, R9 and R10 are methylene group, and
R11 and R12 are prolylene group.

[0051]The most preferable example of the radical polymerization initiator
(I) is VPS-1001, a polymer azo-based polymerization initiator available
from Wako Pure Chemical Industries, Ltd. This polymer azo-based
polymerization initiator corresponds to a compound having the above
General Formula (I) wherein m is an integer of 135, number-average
molecular weight is 70,000 to 90,000, molecular weight of
polydimethylsiloxane moiety is about 10,000, and number of moles of azo
group per 1 g of VPS-1001 is about 0.09 mmol/g.

[0052]As an analogue of VPS-1001, VPS-501 is available from Wako Pure
Chemical Industries, Ltd, which corresponds to a compound having General
Formula (1) wherein m is an integer of 68, number-average molecular
weight is 30,000 to 50,000, and molecular weight of polydimethylsiloxane
moiety is about 5,000. However, VPS-501 cannot produce micron polymer
particles with a low molecular weight without causing flocculation and
aggregation.

[0053]The reason for this is that since the molecular chain of the
polydimethylsiloxane moiety of VPS-501 is shorter than that of VPS-1001
and the molecular weight of the polymer azo-based polymerization
initiator itself is small, VPS-501 offers poor surface activity
(dispersivity as dispersive resin) upon dispersion polymerization,
causing flocculation and aggregation of resultant polymer particles.

[0054]For this reason, it is preferable to employ VPS-1001 having a long
polydimethylsiloxane chain as the polymer azo-based polymerization
initiator. When VPS-501 is used, micron polymer particles with a low
molecular weight cannot be obtained, and in addition, polyhedral polymer
particles cannot be obtained.

[0055]Thus, the radical polymerization initiator (I) used in the present
invention preferably has a number-average molecular weight more than
50,000. The upper limit of the number-average molecular weight of the
radical polymerization initiator (I) is not particularly limited, so long
as the radical polymerization initiator (I) can dissolve in
super-/sub-critical fluid or a monomer to such an extent that it can at
least serve as a polymerization initiator for the monomer in the
supercritical fluid and/or subcritical fluid. Preferably, it has a
number-average molecular weight of 70,000 to 90,000.

[0056]The azo group moieties of the main chain of the radical
polymerization initiator (I) having General Formula (1) are decomposed
during heterogeneous polymerization of a monomer in super-/sub-critical
fluid, forming a block copolymer in which polymers composed of the
monomer are bonded to the decomposition sites. The block copolymer
surrounds the insoluble polymer, formed by heterogeneous polymerization
of the monomer, for stabilization. In this way polyhedral polymer
particles are formed.

[0057]When the polymer particles surrounded by polydimethylsiloxane
segments of the block copolymer undergo rapid pressure reduction,
polydimethylsiloxane segments with low glass transition points are firmly
bonded together. Thus it is contemplated that formation of roughened
polyhedral polymer particles is favored over formation of spherical
polymer particles with smooth surface. As used herein "polyhedral polymer
particles" refer to particles obtained by forming multiple flat and
concave portions on spherical particles As a specific example, particles
as shown in the SEM image of FIG. 4 can be exemplified.

[0068]A representative example of the carboxylic acid ester monomer is
vinyl acetate, and a representative example of the halogenated vinyl
monomer is vinyl chloride. These monomers may be used in combination.

[0069]Upon production of inventive polymer particles, it is preferable to
use radical polymerization initiator (II) in an amount of 0.005 mol to
0.05 mol, more preferably 0.01 mol to 0.03 mol, per 1 mol of total
monomer.

[0070]Upon production of inventive polymer particles, it is preferable to
use a surfactant precursor having General Formula (1) in an amount of 0.5
parts by mass to 10 parts by mass, more preferably 2 parts by mass to 8
parts by mass, per 100 parts by mass of a monomer.

[0071]Upon production of inventive polymer particles, it is preferable to
use a monomer in a volume of 5 parts by volume to 50 parts by volume,
more preferably 10 parts by volume to 30 parts by volume, per 100 parts
by volume of a high-pressure cell used as a reaction vessel during the
production process.

[0072]In the inventive production method, the temperature of the
high-pressure cell or reaction vessel is 40° C. to 80° C.,
more preferably 55° C. to 70° C. If the temperature is
below 40° C., it results in low polymerization rate and thus in
prolonged polymerization time. If the temperature is above 80° C.,
it becomes necessary to apply extremely high pressure and thus a heavy
load is undesirably placed on the cell. It is preferable to keep the cell
pressure at 20 MPa to 50 Mpa, more preferably to 30 MPa to 40 MPa. If the
cell pressure is below 20 MPa, precipitates result during polymerization,
making resultant particles irregular in shape. If the cell pressure is
above 50 MPa, a heavy load is undesirably placed on the cell. The
stirring speed during polymerization is 100 rpm to 900 rpm, more
preferably 200 rpm to 800 rpm. If the stirring speed is below 100 rpm or
above 900 rpm, it undesirably results in particle flocculation.

[0073]The number-average molecular weight (Mn) of the polymer obtained
with the inventive production method is often about 5,000 to 8,000, and
molecular weight distribution (Mw/Mn) is 2.0 to 2.5.

[0074]The particle diameters of polymer particles (polyhedral particles)
obtained with the inventive production method can be adjusted to fall
within 1 μm to 4 μm, and polydispersivity thereof is 1.2 to 2.9.
Thus the polymer particles (polyhedral particles) have uniform particle
diameters.

EXAMPLES

[0075]A commercially available styrene monomer was washed with a 5 wt %
aqueous sodium hydroxide solution, and a radical polymerization inhibitor
was removed by reduced-pressure distillation. While stirring the purified
styrene monomer with a stirrer, it was bubbled with nitrogen gas for 15
minutes to remove oxygen therefrom. 2.2 mL of the purified styrene
monomer subjected to deoxidization was then added to 100 mg of a polymer
azo-based polymerization initiator (VPS-1001, Wako Pure Chemical
Industries, Ltd.), and stirred at room temperature until the polymer
azo-based polymerization initiator was completely dissolved. A
high-pressure cell with an inner volume of 10 mL was purged with nitrogen
gas for removal of oxygen, 95 mg of azobisisobutylonitrile (Wako Pure
Chemical Industries, Ltd.) and the above styrene solution of polymer
azo-based polymerization initiator were added in the cell, and the cell
was sealed hermetically. Subsequently, carbon dioxide which had been
liquidized by a double plunger pump equipped with a cooler was fed into
the cell to a cell pressure of about 18 MPa at 35° C., and then
the valve was closed. After heating the cell to 65° C., liquidized
carbon dioxide was again fed into the cell to a cell pressure of 40 MPa.
Reaction was effected at 65° C. for 24 hours under stirring at 300
rpm. Thereafter, the cell was cooled to room temperature, and carbon
dioxide was gradually discharged until the cell pressure reached to
normal pressure. In this way particles were obtained in the form of while
powder. The obtained particles had a number-average molecular weight (Mn)
of 7,140 and a molecular weight distribution (Mw/Mn) of 2.17. FIG. 1
shows an SEM image of the particles. The particle diameter (Dn) was 1.02
μm and dispersivity (Dw/Dn) was 1.20.

[0083]A calibration curved was then prepared using monodispresed styrene
standard samples. Thereafter, the number-average molecular weight (Mn)
and weight-average molecular weight (Mw) of obtained toner were
calculated from the distributions.

<Measurement of Dispersivity>

[0084]For calculation of dispersivity of particle diameter, 100 particle
images were randomly sampled from an SEM image, their diameters were
measured, and dispersivity was calculated using the following equation:

[0086]Polymer particles of Comparative Example 2 were prepared as in
Example 1 except that azobisisobutylnitrile was not added. Polymerization
hardly occurred and observation of the obtained polymer particles failed
since they were dissolved in unreacted styrene monomers.

COMPARATIVE EXAMPLE 3

[0087]Polymer particles of Comparative Example 3 were prepared as in
Example 1 except that a homopolymer of dimethylsiloxane (number-average
molecular weight (Mn)=46,000, molecular weight distribution (Mw/Mn)=1.96)
was used instead of VPS-1001. The obtained polymer particles had
irregular shape.

[0088]To study how stirring speed influences the generation of polymer
particles, polymer particles were prepared at different stirring speeds
(700 rpm and 1,000 rpm) under the same condition as in Example 1. The
polymer particles prepared at a stirring speed of 700 rpm had a
number-average molecular weight (Mn) of 7,990, molecular weight
distribution (Mw/Mn) of 2.04, particle diameter (Dn) of 1.65 μm, and
dispersivity (Dw/Dn) of 1.99. The polymer particles prepared at a
stirring speed of 1,000 rpm had a number-average molecular weight (Mn) of
8,470, molecular weight distribution (Mw/Mn) of 1.96, particle diameter
(Dn) of 1.13 μm, and dispersivity (Dw/Dn) of 2.70. At 1,000 rpm,
particle aggregation was observed. FIG. 3A shows an SEM image of the
polymer particles prepared at a stirring speed of 700 rpm, and FIG. 3B
shows an SEM image of he polymer particles prepared at a stirring speed
of 1,000 rpm.

EXAMPLE 3

[0089]After washing a commercially available 4-chlorostyrene monomer with
5 wt % aqueous sodium hydroxide solution, a radical polymerization
inhibitor was removed by reduced-pressure distillation. While stirring
the purified 4-chlorostyrene monomer with a stirrer, it was bubbled with
nitrogen gas for 15 minutes for oxygen removal therefrom. 2.2 mL of the
purified 4-chlorostyrene monomer subjected to deoxidization was then
added to a polymer azo-based polymerization initiator (VPS-1001, Wako
Pure Chemical Industries, Ltd.) to a concentration of 4 wt %, and stirred
at room temperature until the polymer azo-based polymerization initiator
was homogenously dispersed. A high-pressure cell with an inner volume of
10 mL was purged with nitrogen gas for removal of oxygen, 0.263M of
azobisisobutylonitrile and the above 4-chlorostyrene solution of polymer
azo-based polymerization initiator were added in the cell, and the cell
was sealed hermetically. Subsequently, carbon dioxide which had been
liquidized by a double plunger pump equipped with a cooler was fed into
the cell to a cell pressure of about 18 MPa at 35° C., and then
the valve was closed. After heating the cell to 65° C., liquidized
carbon dioxide was again fed into the cell to a cell pressure of 40 MPa.
Reaction was effected at 65° C. for 24 hours under stirring at 300
rpm. Thereafter, the cell was cooled to room temperature, and carbon
dioxide was gradually discharged until the cell pressure reached to
normal pressure. In this way particles were obtained in the form of while
powder. The obtained particles had a number-average molecular weight (Mn)
of 46,400 and a molecular weight distribution (Mw/Mn) of 3.49. The
particle diameter (Dn) was 1.53 μm and dispersivity (Dw/Dn) was 2.31.

EXAMPLE 4

[0090]Polymer particles of Example 4 were prepared as in Example 1 except
that the reaction temperature was changed to 55° C. The obtained
polymer particles had a number-average molecular weight (Mn) of 16,300,
molecular weight distribution (Mw/Mn) of 2.71, particle diameter (Dn) of
1.34 μm, and dispersivity (Dw/Dn) of 2.45.

EXAMPLES 5 TO 7

[0091]Polymer particles of Examples 5 to 7 were respectively prepared as
in Example 1 except that the added amount of the polymer azo-based
initiator VPS-1001 was changed as shown in Table 1, which also lists the
physical properties of the obtained polymer particles.

[0092]Polymer particles of Comparative Example 4 were prepared as in
Example 1 except that the polymer azo-based polymerization initiator
VPS-501 was used instead of the polymer azo-based polymerization
initiator VPS-1001. The obtained polymer particles had a number-average
molecular weight (Mn) of 7,020, molecular weight distribution (Mw/Mn) of
2.31, particle diameter (Dn) of 0.805 μm, and dispersivity (Dw/Dn) of
1.20. However, the particles were flocculated and aggregated and thus
could not be taken out as discrete particles. FIG. 5 shows an SEM image
of the polymer particles.

COMPARATIVE EXAMPLE 5

[0093]Polymer particles of Comparative Example 5 were prepared as in
Example 1 except that methyl methacrylate monomer was used instead of
styrene monomer and that the polymer azo-based polymerization initiator
VPS-501 was used instead of the polymer azo-based polymerization
initiator VPS-1001. The obtained polymer particles had a number-average
molecular weight (Mn) of 21,800 and molecular weight distribution (Mw/Mn)
of 2.09. However, the particles were flocculated and aggregated and thus
could not be taken out as discrete particles. FIG. 6 shows an SEM image
of the polymer particles.